Q.Describe the behaviour of a wire under gradually increasing load.
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Start your 14-day free trial to unlock the full solution →As the load on a wire is increased gradually, its stress-strain graph passes through a linear (Hooke's-law) region, an elastic limit, a yield point, a plastic region, and finally fracture.
Suppose a wire is suspended and loaded with gradually increasing weights, and a graph of stress (load per unit area) against strain (fractional extension) is drawn. The curve shows the following stages:
1. Proportional region (O to A). For small loads the stress is directly proportional to the strain; the graph is a straight line through the origin and the wire obeys Hooke's law. Point A is the proportional limit. The slope of this line gives Young's modulus.
2. Elastic limit (A to B). A little beyond A, the graph is no longer straight, but the wire is still elastic — if the load is removed it returns exactly to its original length. Point B is the elastic limit, the maximum stress up to which the wire behaves elastically.
3. Yield point and plastic region (B to D). If the load is increased past B, the wire no longer returns to its original length on unloading — it acquires a permanent (plastic) set. The point at which large permanent deformation sets in is the yield point, and the corresponding stress is the yield strength. In the plastic region the strain increases rapidly for little extra stress.
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